I've done work for netversant and they are not too bad. They are all going to provide you some level of support but you get what you pay for.
You can try:
Nextira One
Sprint
Pyramid Communications
Netversant
Signet6
What you want to look for is someone with a local presence. Some companies will sign a maint contract and then sub the work out locally. It is not in the best interest of the sub to provide you with immediate help, since they are not getting the monthly check. Create reasonable SLA (Service Level Agreements) and have whomever you go with, back them with monetary fines.
We used Shared Technologies for a little while, when I was working in Austin, but I knew their local techs. There were a couple times when they did not meet their SLA and they told us we did not have to pay that month's bill. Shared has a cool online ticket opening system that I really liked.
But think about what you realistically need. Do you need 24X7? Most people don't. Do you need business hour coverage? Most people go with this and then set SLA's for after hours. You have to pay Time and Material charges, but you have to look at how many times you needed help after hours over the past year.
Look at long term contracts (2+ years). You will save money here and see if you can still squeeze a loyalty package from Nortel and get a succession upgrade for free (minus hardware and labor).
And most of the techs here can give real good input on all vendors. some of it will be nice and some will be not so nice.
my opinions
John