I would highly advise against this concept, and I'll explain why. When a phone (or server) dialout to the PSTN (Telephony) network, there are various delays to be considered. The over all time considerations are as following: 1. Dial Delay - The time it takes the first

SIP INVITE

to be acknowledged by the carrier for processing by a

SIP 100 Processing

response - normally, under 200ms, assuming the carrier is local. 2. Pre-Dial Tone Delay (aka: PDD) - The time it takes from the

SIP 100 Processing

event to the

SIP 180 Ringing

or

SIP 183 Ringing

event - this time various significantly from destination to destination and from network to network. For example, in the US this time can be under 1500ms, while in other countries, this can be also 10sec. 3. Ringing Tone duration - The actually ringing tone that started, once the

SIP 180/183 Ringing

event had triggered. This means that when you set a ring timeout for 25 seconds, depending on the geography and targets - you may be automatically removing around 20% of your targets, due to various network related issues - such as roaming network, local network congestion, transition between local exchanges and more. In most dialer scenarios I've seen over life (and I've been doing this for over 25 years) - the recommended dialing timeout should be set to 60 seconds, unless you use localized carriers and direct interconnects.