I'm running my UMTS modem with networkmanager and (happy to have it) modemmanager.
Selecting my provider settings with the help of mobile-broadband-provider-info gets all settings right exept the nameserver entries. The provider info has two nameserver entries, still nm-config-editor in the mobile broadband properties subdialo, IPv4 setting tab chooses "automatic (ppp)" instead of "automatic (ppp) adresses only". If I change this option, the nameserver input field get editable, but is empty. I would expect it to be active and aready filled with the namservers.

With the above options set correct for my provider (non-ppp DNS servers), resolv.conf is written wrong, having two nameserver entries from ppp and one from the DNS servers setting. I checked the resolvconf files, and both NewtworkManager and pppd set nameserver entries. In the resulting resolv.conf file the pppd nameserver entries come first, but as they are not valid/reachable I get horrible long resolv times due to getting through all nameserver with timeouts.
Looking at the command line of pppd:

/usr/sbin/pppd nodetach lock nodefaultroute user ppp@a1plus.at ttyUSB0 noipdefault noauth usepeerdns lcp-echo-failure 0 lcp-echo-interval 0 ipparam /org/freedesktop/NetworkManager/PPP/4 plugin /usr/lib/pppd/2.4.4/nm-pppd-plugin.so

I see "usepeerdns" which is IMO wrong in this situation, as NM is already providing nameserver, and I chose to override the ppp nameservers.

Regards
Patrick