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I have a customer with a 2811. They are going to be expanding their network to exceed 36 phones. So, we need to move them into another system.

We are currently planning the 48user bundle with the 12 FXO ports. Because of their location, they cannot get PRI or ISDN at a reasonable rate. So, they are stuck with POTS. They may need as many as 18 POTS lines coming in.

Does anybody have any good ideas for this? It has been suggested that we buy an ISR, but the price goes up exponentially. A few other suggestions:
1. use their existing 2811 to route outbound calls. Most of the central routing would be done on the UC, but the UC could be configured to route calls through the 2811. would we use SIP for this? How would the UC and the 2811 be connected?
2. Buy two UCs and put them back to back. Maybe the second has 8 fxo and an expansion slot for other technologies.
3. Use SIP over their internet provider. Being unmanaged, the quality would decrease, but if it was only for outbound calls during times when the POTS lines were maxed, maybe it would be acceptable.

Any comments or other suggestions would be great.

Thanks,
Ben