Voice over IP as far as my research finds has a good range of value in adsense. With the lowest paying keyword being (voip buster $2.12) to the highest being (voip solutions $17.46) Here is the entire list of voip keywords.

voip buster $2.12 voip router $2.75 linksys voip $2.80 free voip $2.92 voip calls $2.94

voip gateway $3.41 voip info $3.52 voip review $3.72 optimum voice $3.75 cheap voip $3.80 $3.85 asterisk voip $3.88 voip $4.29 ip phone $4.61 voip offer $4.89

voice over ip $5.12 voip software $5.19 voip provider $5.20 voip call $5.37 cisco voip $5.46 voip technology $5.49 best voip $5.55 voip telephone $5.91

ip phones $6.00 ip relay $6.05 voip telephony $6.24 voip telephones $6.24 compare voip $6.29 residential voip $6.40 ip telephony $6.49 voip service providers $6.57 broadband voip $6.67 voip phone $6.67 voip service $6.94

voip providers $7.04 voip service provider $7.10 voip phones $7.54 voip phone service $8.19 voip services $8.42 voip telephone service $8.94 voip com $9.15 voip phone system $9.45 voip calling $10.30 business voip $11.51 voip pbx $11.69 voip systems $11.73 voip business phone system $12.38 voip solution $12.61 small business voip $13.01 voip phone systems $13.03

voip solutions $17.46

There you have it. Of course these figures will change because of bids, so this information is only to provide you with a frame of reference.

See my other Hubs for more categories.  I will be adding more soon.