I really can't see anywhere in the Terms where it says it's not allowed http://adwords.google.com/support/aw/bin/static.py?page=guidelines.cs&topic=all Where have you seen that it isn't? the only direct reference to it that I've found is the one I posted above, which categorically says it is.
It's NOT against the TOS, but I can't see the point in doing it. Either way it'll negatively affect your CTR and end up costing you money through lower QS (either by people not clicking because they call you instead, or because they don't click or call you because instead of offering free delivery in your ad you've used the valuable space on a phone number).
I am one advertiser who has tested campaigns with/without telephone numbers and in my experience having a telephone number does seem like a relatively good idea initially - obvious benefits being less clicks = less cost and direct contact to our sales team. However it was noted a lower CTR was achieved with adverts displaying telephone numbers as opposed to those which didn't - and a very small amount of traffic on those telephone numbers displayed - which was not enough to warrant displaying numbers over relevant ad information. Although this could be very industry specific - I compete in an industry where customers want a rapid service - not necessarily information, and as mentioned above, believed the telephone number would be a good trial.
> obvious benefits being less clicks = less cost and direct contact to our sales team. But how do you know the calls were a result of the ad? Agree results will vary depending on industry. On the whole however, as JHardy said, I'd rather put those dozen characters needed for a phone number to better use.
I think it is better not to put Tel numbers on the ads because as they said, you will gain less clicks. With that, the main purpose is to market both your website and your sales.
By using a different telephone number for each ad group which is not advertised in any other media form to track calls from the adverts which provides accurate stats on the traffic generated from the listings.