Voicemail peak-hour oversubscription ratio (48:1)

If I’m a phone company and I have 100 subscribers, and every one of them has voicemail, how many people will be calling into the voicemail system at any one time?

Back in the old days, they’d provision trunks into the voicemail system between the Class-5 switch and the voicemail system. They’d have to know how many trunks to provision to let all the subscribers both receive voicemails, and call in to check the voicemails.

I don’t have any good answers here. But I did a quick study using data from logs on a couple of service providers.

These are new-fangled VoIP service providers. So for them, a media/application server supports:

  • Voice mail
  • Automated Attendant
  • Music on Hold
  • Call Center (hold queues)

However, the server can email voicemails somewhere else. So when people check their voicemail, they may not be checking over the phone. This is different than the old-school voicemail systems of 2001, and affects the utilization.

Just as an example, one service provider I checked had a 48:1 oversubscription ratio: i.e., for each 48 subscribers, there’s one RTP stream at peak. They also had about 0.0001 calls-per-second-per-subscriber.

(I say “at peak” because I’m interested in building a network that supports the peak, “busy-hour” load. If I design for the average, I’ll have problems when the load goes above that average.)

Does this ratio apply to you? All of these things could affect it:

  • How many of your users actually buy your voicemail service?
  • …check their voicemail over the phone?
  • …get all their voicemail via email, and listen to it that way?
  • …use automated attendant?
  • …put callers on hold, and get on hold music?
  • …use call centers? And how long are callers waiting in queue?

As another note, it appears that the average duration of voicemail is in the 25-30 second range. This is true at two service providers, both in NFL cities.

Update — A residential service provider reports around a 160:1 oversubscription ratio just for basic voicemail. (I.e., One concurrent call for every 160 subscribers.) A few of them do get voicemail via email.

Someone familiar with voicemail software development told me they expect over 200 subscribers per concurrent call for traditional voicemail.

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