Session Based Number Pools
A session based number pool is a set of tracking numbers on your website, where each visitor is given their own number for their visit. Call Bullet holds the number while the visitor browses, then recycles it once it has been free long enough. The right pool size depends on how many visitors browse at the same time.
Last updated · Call Bullet
What is a session based number pool?
It is the engine behind visitor level call tracking. Instead of one number per channel, your website draws from a pool of numbers and assigns one to each visit. When that visitor calls, the number tells Call Bullet which visit — and therefore which source — the call belongs to.
See the visual website number pools guide for the complete journey through Call Bullet. The technical concepts are also covered in dynamic number insertion explained and visitor level call tracking.
How long is a number held for each visitor?
A number is held while the visitor is active and for a period afterwards, so someone who reads a page, finds a pen and then rings is still matched correctly. Only after that does the number return to the pool.
The balance matters: holding numbers too briefly risks crediting a call to the wrong visitor; holding them too long means you need a bigger pool.
How do you size a number pool?
Size for your busiest moment, not your daily average.
- Find your peak concurrent visitors in GA4 — the most people on the site at once during a busy hour.
- Decide which traffic gets swapped: all visitors, or paid traffic only.
- Start with enough numbers to cover that peak, plus a margin for spikes such as a radio advert or promotion.
- Watch Call Bullet's pool usage for a few weeks and add numbers if the pool runs close to full.
| Site type | What drives pool size | Practical approach |
|---|---|---|
| Small local business | Low concurrent traffic | A small pool, reviewed after a month |
| Busy multi-service site | Many visitors at peak times | Size to peak concurrency plus margin |
| Paid-only swapping | Paid visitors only | Smaller pool than swapping everyone |
| Several locations | Traffic per location | One pool per location with local codes |
What happens when the pool runs out?
If every number is in use, a new visitor has to share a number with someone else. Calls still reach you — no call is lost — but a shared number makes it harder to know which visit a call belongs to, so attribution accuracy drops.
That is why it is worth sizing for peaks and checking usage. Extra numbers can be added at any time on Buy numbers.
Why use one pool per location?
Callers trust local numbers. A business with branches in Leeds and Manchester should give visitors to the Leeds pages 0113 numbers and visitors to the Manchester pages 0161 numbers. Call Bullet creates website pools by location for this reason, while fixed placements such as print or Google Business Profile are managed separately.
Should you swap for all visitors or just paid traffic?
Swapping for everyone shows calls from every source, including organic search and social, but needs a larger pool. Swapping for paid traffic only keeps the pool small and still measures your ad spend. Many businesses start with everyone if traffic is modest, because organic calls are often the surprise in the data — see call tracking for SEO.
How much does a pool cost?
Cost is the number of numbers in the pool multiplied by the monthly number price, plus call minutes. Current prices are on Pricing. Because numbers are recycled between visitors, even a modest pool can cover a busy site.
Key takeaways
- A session based pool assigns each visitor their own number for their visit.
- Numbers are held after the visit, then recycled.
- Size the pool for peak concurrent visitors, not daily totals.
- A full pool never loses calls but reduces attribution accuracy.
- Use one pool per location with local area codes.
Frequently asked questions
How many numbers should my pool have?
Enough to cover the most visitors on your site at the same time, plus a margin for spikes.
Are calls lost if the pool is full?
No. Calls still forward to you; only the attribution becomes less certain.
Can I add numbers later?
Yes. Add numbers to a pool at any time from Buy numbers.
Why not just use one number per channel?
A channel number can't tell you campaign, keyword or landing page. A pool can, per visit.
Do I need separate pools for different towns?
If you serve several locations, yes — local area codes reassure callers in each place.
Does the pool include my Google Business Profile number?
No. Profile, print and other fixed placements use their own numbers outside the website pool.
See which marketing makes your phone ring
Start with one UK tracking number or a full website pool. Every call is recorded, transcribed and traced back to its source.