Theoretical limit on routers is 254 however there are other limiting factors that will hamper some operations.
If your traffic is low or limited and mainly used for accessing the Internet for mail and such, then these routers can handle that. I also have a 24 port switch on the back end of mine with about 10-15 wired devices going at various times. Some routers handle low end VoIP like MagikJack and Vonage, then some don't. For bigger VoIP class phones you definitely need a business class router that can handle that or one specificly designed for business class VoIP, i.e. Cisco and the like.
I would check with your VoIP provider and ask them what they require and support for external routers and what they might recommend.
Is your current set up working at this time? Are there any problems your seeing?
I 1Mb line might be ok however if you have a lot of computers accessing the Internet all at once, might need a bigger pipe. Most ISP services avg now days from 5mb down to 50mb down and even higher. I have a 50/2 service at home. However I game too.
I would contact Dlinks enterprise people and talk to them about your set up and see if they have any recommendations on a particular router they might have that would be best suited for your needs. Again, DIR and DGL routers are mainly designed for home use and small business with a few devices connected and general internet connections. Anything specialized beyond that would probably not be in your best interests.
Let us know if you have any more questions or need more help.